Mormon Pioneer from Little Dell to the bathrooms is easy. Cross the road and if you can manage a short hill, then it's pretty cake to Affleck park too. It gets steeper after that but would be good for progression, and when you've had enough, turn around and coast back down.

Millcreek Pipeline in SLC is a classic, relatively flat beginner ride that goes to a great overlook above city and features a mix of buff singletrack, a couple berms, a few switchbacks if you start from the top, and even a few easy stretches of rock.

Easy and Flat.

The BST in the north part of the SLC valley is not at all flat. Technically is not too difficult, but there is alot of climbing. Most of the climbs are pretty short (less then 30 min) but its not rideable by a beginner unless they are pretty fit.

Round Valley, Millcreek Pipeline, and the lower parts of Mormon Pioneer are good suggestions. I would add Muller Park in bountiful to your Radar. Its not flat but it has a easier grade then most trails in the area,